There are several known techniques that can be used to recover gold and silver from aqueous solutions. Metal cementation has been used for this purpose for about one century in the cyanide industry. The Merrill-Crowe process (N.P. Finkestein, The chemistry of the extraction of gold from its ores, Gold metallurgy in South Africa, R.J. Adamson, Chamber of Mines of South Africa, Johanesburg, 284-351, 1972) uses a zinc powder to recover these metals from a cyanide solution while aluminum powder is more practical for an acid thiourea solution (A. Van Lierde, P. Ollivier and M. Lesoille, Developpement du nouveau procede de traitement pour le mineral de Salsigne, Ind. Min. Les tech., 1a, 299-410, 1982). Commercial implementation of the carbon-in-pulp and electrolysis occurred in the 1970's with some improvement of the Zadra work (Anon., Homestake uses carbon-in-pulp to recover gold from slimes, World Min., 27:12, 44-49, 1974) and is actually widely used in the gold cyanide industry.
New extraction systems are being applied (J.E. Barnes and J.D. Edwards, Solvent extraction at INCO's Acton Precious Metal Refinery, Chem. Ind., 5, 151-155, 1974; L.R.P. Reavell and P. Charlesworth, The application of solvent extraction to platinum group metals refining, ISEC 80, Liege, Belgium, 1980; R.F. Edwards, Selective solvent extractants for the refining of platinum metals, ISEC 77, CIM special volume 21, 24-31, 1979; G.B. Harris and R.W. Stanley, Hydrometallurgical treatment of silver refinery anode slimes, 10th Int. IPMI Conf., Lake Tahoe, Nevada, U.S.A., June 9-12, 1986) or proposed (G.P. Demopoulos, G. Pouskouleli and G.M. Ritcey, A novel solvent extraction system for the refining of precious metals, ISEC 86, West Germany, 1986) to recover gold and silver from precious metals refining solutions. Gold is recovered from the loaded organic by cementation, reduction with oxalic acid, hydrolytically stripped or reduced with sulphur dioxide or hydrogen.
In the development of new leaching processes to recover gold and silver from ores and concentrates, thiourea is one of the most promising reagents with its low toxicity. Having the capacity to treat mild refractory materials, thiourea leaching also saves the costs related to cyanide destruction. Its efficiency to treat a gold chalcopyrite concentrate that can not economically be treated with cyanidation was demonstrated (G. Deschenes and E. Ghali, Thiourea leaching of a gold chalcopyrite concentrate and chalcopyrite residue, CIM Ann. Conf. Met., Toronto, Ontario, Aug. 19-23, 1986). Commercialization of the thiourea process will involve development of the last step of the technique, i.e. recovery of gold (and silver) from solutions.
Different methods can be applied to recover gold and silver from an acidic thiourea solution (G. Deschenes, Literature survey on the recovery of gold from thiourea solutions and the comparison with cyanidation, CIM bull.:11, 1986). These are cementation, adsorption on activated carbon, ion exchange resins, solvent extraction and electrolysis. Raising of pH up to above 6.5 by addition of an alkaline reagent is also proposed causing precipitation (T. Odaka et al, Method of recovering gold and silver from aqueous solutions containing thiourea, gold and silver, Japanese Pat. 60-103138, June 1985). For cementation, Bodson patented the use of aluminum powder (F.J.J. Bodson, Procede de recuperation de l'argent et eventuellement de l'or contenus dans une matiere de depart solide; Canadian Pat. 1,090,141, Nov. 1980) to recover gold and silver: this procedure provides high extractions but the quality of the cement is sometimes poor. The cementation use of lead powder is inefficient in sulfuric solutions while zinc powder is inappropriate in acidic thiourea liquors. The use of activated carbon (R. Schulze, Hydrometallurgical winning of precious metals using thiourea, German Pat. DE 3401961, Aug. 1984) has two serious drawbacks: adsorption of thiourea onto carbon and difficult stripping of gold and silver. The suggested way to effectively recover gold from the carbon is to burn the loaded material but that renders the operation more expensive. Thus, there is a large unknown in thiourea desorption and carbon conditioning.
Acid cationic ion exchangers (resins) resulted in good recovery of gold with lower thiourea loading and easier stripping than activated carbon. This could be used as a purification step in thiourea processing. Solvent extraction also provided a selective and efficient recovery of gold with an easy stripping.
Raising of the pH up to above 6.5 to recover gold and silver from thiourea solution (T. Odaka, op. cit.) is not a suitable method since at such pH thiourea is less stable and decomposes. Also, the acid consumption related to this method is prohibitive because if the leach liquor has to be recycled the pH must be readjusted to 1-2. The end product (precipitate) is not expected to have a high grade.
Electrolysis was used to recover gold and silver from thiourea strip solution on a pilot plant scale (A.I. Maslii and R.Y. Bek et al, "Pilot Test and Implementation of Electrolytic Gold Extraction From Commercial Regenerated Products", TSVETN. Met. 14:18: p. 79-81, 1973). Thiourea is used in the circuit to strip a loaded anionic resin. A two-stage electrolysis has to be performed to produce high recoveries and special cell designs have to be used.
Pressure reduction potentially is an attractive avenue to gold and silver recovery from such solutions because of its selectivity and its ability to precipitate precious metals. In certain aqueous solutions, pressure reduction under hydrogen atmosphere was previously performed to recover platinum, rhodium, palladium (M. Findlay, The use of hydrogen to recover precious metals, Precious Metals 1982, M.I. El Guindy, Pergamon Press, Toronto, Ontario, 477-501, 1983) and silver (A.H. Webster and J. Halpern, Homogeneous catalytic activation of molecular hydrogen in aqueous solution by silver salts. III. Precipitation of metallic silver from solutions of various silver salts, J. Phys. Chem. 61, 1245-1248, 1957). Prior researchers found that chloride, acetate and ethylene complexes of the metals were reduced but it was not possible to precipitate precious metals from cyanide solutions because of the stability of these complexes. In certain organic media, gold and palladium were recovered by the same technique (G. Pouskouleli and G.P. Demopoulos, Direct recovery of precious metals by integration of solvent extraction and hydrogen reduction techniques. Precious Metals 1984, Toronto, Ontario, June 4-7, 1984). See also U.S. Pat. No. 4,654,145 issued Mar. 31, 1987, Demopoulos et al.